1. The thyroid hormone receptor alpha gene is alternatively spliced to generate the alpha receptor and a variant, which does not bind thyroid hormones. An additional ` like receptor, Rev-erbAalpha, is transcribed on the opposite strand overlapping the unique variant sequences. The overlapping region of these two transcripts can form a sense-antisense hybrid and thus modulate a level of variant with concomitant increase in production of the alpha receptor. Our results showed that transcription of these two genes from the erbA alpha genomic locus is independent of each other, since the level of variant is very high at any developmental stage, the level of receptor is very low with transient increase 15 days after birth and the Rev-erbA alpha is high only in the adult tissues. 2. We are interested in the precise mechanism by which thyroid hormone receptor and other ligand activated-nuclear receptors induce or repress transcription of genes in response to a ligand. We and others have demonstrated that at least part of this mechanism involves the ability of the thyroid hormone receptor to heterodimerize, with another hormone receptor. Several questions arise. What is a role of the ligand, one or both, in a heterodimer formation? Can ligand binding affect the affinity and specificity of a heterodimer binding to different response elements? To answer these questions and study various kinetic parameters we have begun to purify thyroid hormone receptor. We have made plasmids expressing a fusion protein, in which the N-terminus of the receptor is fused with polyhistidine. Baculovirus and E.coli expression systems have been employed. A nickle column has been used to purify overexpressed protein.